The invention relates to a device for monitoring an area of coverage on a work tool.
The work tool can be a mobile system, wherein mobile systems of this type can be transport vehicles such as industrial trucks. With systems of this type, the area of coverage is the area in front of the transport vehicle which must be monitored to avoid collisions with persons or objects. The work tools can furthermore be stationary systems such as machines with machine parts that perform dangerous movements. The area of coverage must, therefore, be dimensioned so as to allow the monitoring of regions where endangering movements are carried out. A robot for assembling work pieces is one example of such a work tool. Assembly robots of this type represent a high potential for danger since the operator in general must insert the work pieces manually at the assembly robot.
Protective devices are normally used for securing such areas of coverage, wherein the protective devices shut down the work tool in case of an uncontrolled operator intervention.
Protective devices of this type in particular can be embodied as two-hand control units. These two-hand control units protect the hands of the operator who must actively engage the levers for starting and maintaining a dangerous movement because the operator must keep both hands on the levers and is thus protected against injuries when operating the work tool. The disadvantage of these devices is that other persons in the area of coverage are not protected.
An additional disadvantage with a work tool embodied as assembly robot is that the assembly robot working cycle decreases, thus unnecessarily restricting its availability.
Safety light curtains are also used as protective devices. A safety light curtain of this type is known, for example, from German patent document DE 39 39 191 C3. These safety light curtains consist of several pairs of transmitters and receivers which respectively form a light barrier. Protective devices of this type are used to cover a planar, two-dimensional area to be monitored.
However, protective devices of this type have the disadvantage that numerous such protective devices are required to completely secure an area of coverage around an assembly robot, particularly with respect to access protection and securing of the area. Not only does this represent an undesirably high expenditure, but it also requires an expensive and involved assembly, wherein the available space for installing these devices is frequently insufficient.
It is a further disadvantage that these types of protective devices are not suitable for monitoring areas of coverage around mobile work tools.
Reference European patent document EP 0 902 402 A2 relates to a method for optically monitoring a space, particularly the area around an elevator door, wherein this space is monitored with a camera. Alternatively, several cameras can also be provided by means of which respectively different segments of the space can be monitored. The camera, or each camera, is embodied as a digital camera and is provided with a camera module, an A/D converter, an evaluation unit, a microprocessor, and an image memory. To monitor the space, the evaluation unit performs a pixel comparison between the gray values of an actual image and the gray values of a reference image. In the process, differences are formed between the gray values of the actual image and the gray values of the reference image. The microprocessor also monitors the operational capacity of the camera by detecting whether a basic brightness of the actual image is exceeded or not reached.
The disadvantage of the above-mentioned invention is that the reference image comparison depends on the environment and is thus subject to interference. External light influences in particular can lead to errors in the object detection.